FIELD OF THE INVENTION:
The invention relates to a method for demodulating a voltage that has been amplitude shift keying (ASK) modulated by changing the amplitude between a low level and a high level. This form of modulation is particularly suitable for use during contactless data transmission from a card reader/writer to a smart card. A demodulator is provided for carrying out the method, and also a circuit configuration for a smart card having such a demodulator is described.
Smart cards, which have an electronic chip instead of a magnetic strip, are becoming increasingly important on account of their much higher performance and the associated diverse opportunities for use. Since the smart cards are intended to be operated contactlessly once inserted into a card reader, it is necessary not only to demodulate the voltage that is received on the smart card, but also to produce therefrom the supply voltage required for the chip.
Various modulation methods are known for data transmission from the card reader to the smart card. Besides the on/off keying (OOK) modulation method, modulation by amplitude change between two levels (amplitude shift keying (ASK)) takes place particularly frequently, generally in the range from eight to twelve percent of the total amplitude and using non return to zero (NRZ) coding.
In this context, the demodulator situated on the smart card has particular requirements placed on it, since movements of the card in the transmission field of the card reader have a similar effect to modulation, but need to be distinguished therefrom. The result of this is that the demodulator circuit needs to be able to demodulate, by way of example, 10% level fluctuations of different duration (NRZ coding) for operating voltage fluctuations of different duration which are many times larger than the actual amplitude modulation. Consequently, the dynamic range of the signal that is to be demodulated is also relatively large. In addition, twice the frequency of the medium is superimposed on this signal, since the received signal is first passed via a full-wave rectifier. Published, Non-Prosecuted German Patent Application DE 197 03 967 A1 discloses a receiver with amplitude shift keying. It discloses an ASK receiver having the following features.
A signal reception section for receiving, amplifying and quantizing an ASK signal, a frequency divider for dividing a clock signal by a predetermined value, a counting section for counting the ASK signal at predetermined intervals under the control of the frequency divider, and a comparison section for comparing the count for the counting section with a reference value to ascertain whether the count lies in a predetermined range, in order to determine whether the signal is normal. In addition, a pulse detection section is provided for compensating for errors such as are produced in the comparison section by interference signals which, to some extent, are contained in the signal from the signal reception section. In addition, a counter with a random section is provided for producing a signal that simultaneously satisfies the output signals both from the comparison section and from the pulse detection section by using a clock signal whose speed is n times the transmission rate. Finally, a signal restoration section is provided for restoring the original signal from the signal of the counter with random section.
ISSCC 97, section 17.6 discloses a circuit in which the ASK modulated signal contained in the supply voltage is compared with its delayed signal using a comparator, and the digital signal is obtained therefrom. A disadvantage of this, however, is the level range within which the comparator operates, which is only small. It is then not possible to regulate the supply voltage, since this would result in the loss of the signal that is to be demodulated. Furthermore, level difference measurement is very susceptible to faults.